A subdomain is the section of the web address that is before a domain name and you have almost certainly seen a lot of subdomains while surfing around world wide web. For example, many sites such as Wikipedia have versions in various languages using subdomains - en.wikipedia.org, de.wikipedia.org and so on. The best thing about employing a subdomain is that it can have an independent site and its own records, so you are able to even host it on a different server. The practical use is that you could have a supplementary website, like an e-learning portal for college students in addition to the main school website. If you are using subdomains instead of subfolders, it will be much easier to perform maintenance or to upgrade a particular site, not mentioning that it's going to be more secure to have the sites separate from each other.